One of the major advantages of powder-metallurgical manufacture of components is that it becomes possible, by compacting and sintering, to produce blanks of final or very close to final shape. There are however instances where subsequent machining is required. For example, this may be necessary because of high tolerance demands or because the final component has such a shape that it cannot be pressed directly but requires machining after sintering. More specifically, geometries such as holes transverse to the compacting direction, undercuts and threads, call for subsequent machining.
By continuously developing new sintered steels of higher strength and thus also higher hardness, machining has become one of the major problems in powder-metallurgical manufacture of components. It is often a limiting factor when assessing whether powder-metallurgical manufacture is the most cost-effective method for manufacturing a component.
Today, there are a number of known substances which are added to iron-based powder mixtures to facilitate the machining of components after sintering. The most common powder additive is MnS, which is mentioned e.g. in EP 0 183 666, describing how the machinability of a sintered steel is improved by the admixture of such powder.
U.S. Pat. No. 4,927,461 describes the addition of 0.01 and 0.5% by weight of hexagonal BN (boron nitride) to iron-based powder mixtures to improve machinability after sintering.
U.S. Pat. No. 5,631,431 relates to an additive for improving the machinability of iron-based powder compositions. According to this patent the additive contains calcium fluoride particles which are included in an amount of 0.1-0.6% by weight of the powder composition.
The Japanese patent application No. 08-095649 describes a machinability enhancing agent. The agent comprises Al2O3—SiO2—CaO and has an anorthite or a gehlenite crystal structure. Anortithe is a tectosilicate, belonging to the feldspar group, having Mohs hardness of 6 to 6.5 and gehlenite is a sorosilicate having Mohs hardness of 5-6.
U.S. Pat. No. 7,300,490 describes a powder mixture for producing pressed and sintered parts consisting of a combination of manganese sulfide powder (MnS) and calcium phosphate powder or hydroxy apatite powder.
WO 2005/102567 discloses a combination of hexagonal boron nitride and calcium fluoride powders used as machining enhancing agent.
Boron containing powders such as boron oxide, boric acid or ammonium borate, in combination with sulphur is described in U.S. Pat. No. 5,938,814.
Other combinations of powder to be used as machining additives are described in EP 1985393A1, the combination containing at least one selected from talc and steatite and a fatty acid.
Talc as machining enhancing agent is mentioned in JP 01-255604. Talc belongs to the group of phyllosilcates, consisting silicon tetrahedral layers, enclosing a magnesium hydroxide octahedral layer.
The application EP 1002 883 describes a powdered metal blend mixture for making metal parts, especially valve seat inserts. The blends described contain 0.5-5% of solid lubricants in order to provide low friction and sliding wear as well as improvement in machinability. In one of the embodiments, mica is mentioned as a solid lubricant. These types of powder mixtures, used for production of wear resistant and high temperature stable components, always contain high amounts of alloying elements, typically above 10% by weight and hard phases, typically carbides.
U.S. Pat. No. 4,274,875 teaches a process for the production of articles, similar to what is described in EP 1 002 883, by powder metallurgy including the step of adding powdered mica to the metal powder before compaction and sintering in amounts between 0.5 to 2 weight %. Specifically, it is disclosed that any type of mica can be used.
Further, the Japanese patent application JP 10-317002, describes a powder or a sintered compact having a reduced friction coefficient. The powder has a chemical composition of 1-10 weight % of sulphur, 3-25 weight % of molybdenum and the balance iron. Further a solid lubricant and hard phase materials are added.
Machining of pressed and sintered components is very complex and is influenced by parameters such as type of alloying system of the component, sintered density of the component and size and shape of the component. It is also obvious that type of machining operation and speed of machining are parameters which have a great importance of the outcome of the machining operation. The diversity of proposed machining enhancing agents to be added to powder metallurgical compositions reflects the complex nature of the PM machining technology.